In a wireline communication system, a single trunk line will service many different individual users. For instance, a telephony system will often have various trunk lines fanning out from a main control/switching station, and each of these trunk lines then fan out to individual users. Often a trunk line will run from a head-end (control station and switching network) to a service area node. Many different users will be fed to the node and then networked onto the trunk line.
These trunk lines are often fiber optic cables which are capable of carrying a tremendous number of calls in comparison to cables made of metals. These optic cables or lines can carry signals at speeds much greater than conventional metal lines.
Telephony cable lines, whether optic or metal, operate in pairs such that a trunk line will consist of a downstream cable and an upstream cable forming signal line loops for the telephony signal streams to follow. Generally the up and downstream cables are installed along the same route and have the same length, although not necessarily installed in the same trench or on the same utility line. However, these main up and downstream lines generally parallel each other.
A major goal of telephony systems is to supply dependable use to users so that the system may be accessed even during emergencies. To be reliable, the system must have backup in case a line goes down. When a line goes down, a fault is registered within the system indicating that the signal stream cannot be routed through the main signal line loop. A line can go down for many reasons including natural forces causing a break in the line, construction digging into the area where a line is laid and breaking the line, maintenance on the line by the operator, or any other number of occurrences. Accordingly, operators of telephony systems install redundant trunk lines so that the telephony signal streams may be routed through the redundant trunk lines to form an alternate signal line loop. Generally there will be a designated downstream redundant line and a designated upstream redundant line. If the main, or signal line loop downstream line is broken or disfunctional, the head-end will route the signal stream through the redundant downstream line while using the upstream line of the main, or signal line loop. Similarly, the redundant upstream line may be used in a similar manner. In fact, any combination of the four lines may be used by the head-end.
Although any combination of the four lines may be used, it should be noted that the two main lines (down and upstream) of the signal line loop usually follow the shortest path from the head-end to the service node. For reliability reasons, the redundant lines must follow a different path, often making broad detours resulting in much longer lines than the signal loop lines. Therefore, if a line of the signal line loop is near a construction site, for instance, and capable of being damaged by digging at the site, the redundant line will not be affected since its routing is away from the same area.
The longer length of the redundant line naturally delays the time that the signal stream will take to go from the head-end to the service node and back. This presents a problem with time based signaling protocols such as time division multiple access (TDMA) protocols. Delaying the time a signal stream takes to go to the service node and back beyond the delay expected as the signal follows the main signal line loop alters the anticipated position of the signal stream and control information within the stream once the signal stream returns to the head-end. Communication links are lost and an adjustment must take place to align the signal stream from the service node to the head-end to a position in the protocol that the head-end will be expecting the signal stream to be in.
Accordingly, a method is needed in a telephony system having redundant signal lines to automatically detect when a fault in a signal line loop has occurred and compensate for any delays caused by routing the signal stream through the redundant signal line.